As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2009, and are readily familiar and understood to those skilled in the art of aluminium alloys.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
Multilayer tubes, often abbreviated as MLT, consisting of co-extruded polymeric materials, in particular polyethylene or polypropylene tubes bonded with adhesives to a metal core have found many applications in the building industry, for example as sanitary pipes, heating pipes, gas distribution pipes etc., as well as in other industries such as the pharmaceutical, chemical and food industries. Compared to tubes entirely made from a plastics material, multilayer tubes having an aluminium alloy core layer are impermeable to oxygen and have higher strength and heat resistance, as well as a low expansion coefficient. Such multilayer tubes are flexible and easy to install since they can be bent and cut to the desired length on site. They are also preferred to tubes made of metal only, since the plastic inner and outer layers improve the corrosion resistance and serve as noise barrier. They also save weight and have a better formability.
Such multilayer tubes have an outer polymeric material layer, an inner polymeric material layer and an aluminium core layer in the form of a tube for mechanical strength and long time performance. The inner and outer polymeric material layers are generally bonded to the aluminium core layer by means of an outer and inner adhesive layer, respectively.
One of the most critical properties of multilayer tubes in typical heating and sanitary applications is the resistance to constant and sustained internal pressure at increased temperatures. This is generally tested by the standardized internal pressure tests according to ASTM-F1281, ASTM-F1282 or DVGW-W542 in which the tube is kept at 30 bar and 95° C. until the tube fails. From the time until failure, one may extrapolate the expected lifetime of the tube under the conditions encountered, for example, in the sanitary system of a building, i.e. a temperature of 70° C. at 20 bar. A high resistance against sustained internal pressure is especially important if the metal core layer is to be kept as thin as possible, and for tubes with larger diameters.
Aluminium alloys currently used as core layer in multilayer tubing are AA3003- and AA3005-series aluminium alloys.
The AA3003-series aluminium alloy has a chemical composition, in wt. %, of:
Si<0.6Fe<0.7Cu0.05 to 0.20Mn 1.0 to 1.5Zn<0.10,impurities <0.05 each, total <0.15, balance aluminum.
This aluminium alloy has previously been used in multilayer tubes because it is relatively inexpensive and has sufficient strength. However, it does not perform well under the sustained pressure tests.
Patent document WO-2008/057608-A1 discloses an aluminium alloy product and a multilayer tube having a tube wall made from said aluminium alloy containing, in wt. %:
Si 0.2 to 1.4Fe + Mn 1.1 to 1.8Cu0.15 to 0.5Mg<0.20Ti<0.20Zn<1.5,and other impurities or incidental elements each <0.05, total <0.2, and the balance being aluminum.
This aluminium alloy has an increased resistance against sustained pressure when tested in the ASTM-F1281 creep test after more than 100 hours at 95° C. and a 30 bar internal pressure in comparison with alloys of the AA3003 and AA8006-series.
A need exists for multilayer tubes with an aluminium alloy core and having further improved creep properties, in particular the resistance to constant and sustained internal pressure, ideally at elevated temperature.